This invention relates to a detection device for data relating to the passage of vehicles on a road, which device is to be placed in a groove provided in the upper portion of said road and comprises a first profiled strip having an upper U-shaped cavity and a coaxial piezoelectric cable arranged in the bottom of the cavity of said first profiled strip.
The invention is used for the detection of the passage of vehicles, counting, speed measurements, measurements of vehicle weights (dynamic loads) and the classification thereof, in geographic regions of widely differing, and especially extreme weather conditions (very hot, moderate, or very cold climates).
Patent FR 2 482 340 describes a vehicle passage detection device for a road, its installation method and its use in the detection of speeds.
This device is partly formed by a cable which operates piezoelectrically. This cable is a coaxially shielded cable whose core and sheath are separated by a piezoelectric ceramic material. This cable has an external diameter of a few millimeters and a length of the order of 1 m or more. It is accordingly very long and thin.
To increase the useful life of the piezoelectric cable, facilitate its installation in the road, and ensure a certain reliability of the measurements, this piezoelectric cable is arranged in the bottom of a rigid metal or hard-plastic profiled strip having either a U-shape or the shape of a rectangle of which the upper face has been caved in. This strip is internally filled with a synthetic resin material which can be hardened by polymerization. The object is to provide a rigid device.
In a modification of this device, the rigid profile is surrounded with a moulded envelope of flexible synthetic resin over its free surfaces which are not to come into contact with the vehicles, for example, an elastomer-filled resin which is to absorb the vibrations. This arrangement is realised in the factory.
In either case, the installation method for the device comprises the formation of a groove in the road of dimensions slightly greater than those of the device. Subsequently, the bottom and the sides of the groove are lined with a flexible synthetic resin material which absorbs vibrations, and finally the device is placed in the lined groove so that it remains fixed therein, while it is made to project slightly from the upper surface of the road. If the profile has a U-shape, the upper portions of the legs of the U will project from the upper surface of the road by a few millimeters. Lining of the groove is not absolutely necessary because the device already comprises a moulded envelope of a flexible plastic realised in the factory.
A disadvantage of the prior-art device described above is that its manufacture involves the use of synthetic resin materials, which are organic materials and which are particularly sensitive to temperature variations.
Now it is an object of the invention not only to detect the speed of vehicles, but also to count them, and to detect the weights of moving vehicles (measurement of dynamic charges), and to classify them. The detection and analysis of such data is particularly important for the planning of new roads, new bridges, or road surfaces of any kind, as well as for providing their maintenance.
If organic materials are used for realising detection devices, the measurements are not reliable as regards the detection of vehicle weights, because these measurements depend on climatic conditions. Depending on the situation, these measurements may be non-reproducible, or even impossible in regions where the climatic conditions are extreme.
If it is also necessary to use a plastic material for fixing the device in the road, moreover, the difficulties or impossibilities will manifest themselves in regions where the climatic conditions are extreme (torrid heat, extreme cold, very high humidity, huge climatic differences between the seasons or between day and night, etc.).
Another disadvantage of the known device is that the polymerizable plastic material for filling the U-profile is fragile, has bad ageing properties, and easily disengages itself from the walls of this hard protective profile. This means that the device obtained is not capable of any flexion. In its first embodiment, the device is accordingly fragile in a general sense, especially when used on a deformed or curved road surface; and it is particularly ill adapted to the measurement of moving loads.
In its second embodiment, it is still not flexible, and it is still fragile owing to the use of synthetic resins, but it is better adapted to the measurement of moving loads because of its moulded envelope. In this case, still, it will be difficult and expensive to realise, indeed, the realisation of an added moulded envelope implies the manufacture of a mould, which must be changed whenever those skilled in the art want to modify the length or transverse dimensions of the devices: and indeed the devices must have different lengths depending on the use for which they are designed: measurements on local roads, on motorways, measurements based on half the shaft or the whole shaft of a vehicle, the use of piezoelectric cables of different diameters, etc. Moreover, a mould will deteriorate during use.
Finally, stripping the device from the mould is difficult because of the fragility of the thermosetting resin with which the profile is fired.
A detection device for data relating to the passage of vehicles is also known from the prior-art Patent Application EP 0 231 669, which device is to be inserted into a groove provided in a road and also comprises a piezoelectric cable protected by a hard U-shaped profile, and is embedded in a filling material of the profile. In this second cited document, the profiled strip has a substantially square cross-section and is made from metal, for example, aluminium. The cable is at a given distance from the bottom of the profile and the filling material for embedding the cable is a silican-filled epoxy compound. The sides of the U-profile are provided with an elastomer foam for absorbing longitudinal bending. The assembly is introduced into a groove provided in the road and is again embedded in a silican-filled epoxy compound.
This second cited device has two advantages over the first cited device, i.e.: an improved performance in the measurement of moving loads because it is accurately sensitive to vertical pressures, and an improved resistance to weather conditions because the silican-filled epoxy compound acting as the filling material is especially provided for this purpose. But it also has major disadvantages: the first disadvantage, which was also present in the first cited device, lies in the fact that this second device is not any more flexible, because the silican-filled filling resins are very brittle. The second disadvantage is that it is even more expensive to implement than the second modification of the first device because of its highly complicated structure. Moreover, positioning of the piezoelectric cable at a given distance from the bottom of the profile is difficult to realise because it is not easy to keep the thin, long cable at a well-defined distance during this filling operation.